The present invention relates to very thin layers of electrical insulation that may be used to coat and protect microminiature components and devices that are intended to be implanted in living tissue and/or to maintain electrical leakage of such components/devices within acceptable limits, e.g., less than 1 μA/cm2 when the components and/or devices are submerged in water or salt water. More particularly, the invention relates to the use of alumina or aluminum oxide as a safe, biocompatible, coating material that provides a reliable, protective and insulative layer or coating for components, or devices comprised of components, wherein the insulating layers can be made extremely thin, on the order of microns, yet wherein the electrical leakage through the thin insulative layer (when the coated component or device is implanted or otherwise immersed in a saline solution or in distilled water) is less than about 1 μA/cm2 (or less than about 12.1 nA for an area of 0.075 inches×0.025 inches, corresponding to an area of 0.1905 cm by 0.0635 cm).
The use of alumina as a thick insulator for use with implantable devices has previously been disclosed, for example, in U.S. Pat. Nos. 4,940,858 and 4,678,868 assigned to Medtronic, Inc. In these applications, however, the alumina insulator is very thick and is used only as part of the feedthrough for the implantable device and is often carried by a metal ferrule. Such use of alumina (or other ceramic) as an insulator requires a relatively thick layer. Many materials work well as an insulator when put down in a thick layer, e.g., in a layer thicker than 25 microns (where 1 micron=1×10−6 meter). But all such materials, except as discussed herein, typically leak at a rate greater than about 1 μA/cm2. Applicants invention, as set forth below, uses a nonconductive ceramic, such as alumina, in very thin layers, e.g., less than about 25 microns.
It is also known to use the ceramic alumina as a case material for an implanted device as disclosed in U.S. Pat. No. 4,991,582, incorporated herein by reference. Again, however, the alumina, while comprising a material that is biocompatible (and is thus not harmful to, and is not harmed by, living tissue and fluids wherein it is implanted), is relatively thick, e.g., greater than 25 microns.
A problem with the related art is that the thickness of the insulation needed for implantable devices is typically on the order of about several millimeters thick. None of the related art, to applicant's knowledge, has heretofore achieved an insulating layer with very small dimensions and free of micro-holes. The presence of a micro-hole, or “pin-hole”, destroys the insulating properties which may lead to eventual failure of the implantable device.
Further, some components or devices which need to be implanted in living tissue, such as magnets, are susceptible to extremely high temperatures, i.e., extremely high temperatures may damage or destroy such components. When such components or devices must be implanted, it is important therefore that whatever coating or encapsulating material is used to coat them be one that can be applied without subjecting the component or device to extremely high temperatures. That is, the coating or application process must not subject such components to extremely high temperatures.
It is seen, therefore, that what is needed is a way to utilize a very thin layer of a suitable insulating material, such as alumina (aluminum oxide), zirconia (zirconium oxide), or alloys of alumina and/or zirconia, at relatively low temperatures, as a coating to cover, insulate and/or encapsulate any type of component or device that must be implanted, thereby effectively rendering such coated component or device biocompatible and safe for implantation. In particular, it is seen that what is needed is a very thin insulative coating that can be applied at relatively low temperatures for the purpose of insulating electrical connections on implantable devices and other microminiature devices, or for coating non-biocompatible components (thereby making the coated component biocompatible) wherein the coating can be as thin as about 1/1000 of an inch or less yet still maintain the electrical leakage through the insulator at or below acceptable levels.
The present invention addresses the above and other needs.